linux-kernel - Re: 2.6.22.6 - Discrepancy between running and on-disk kernels

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Message-ID: <54b90fdf0803140908k44bee3a2gbf4187ae40dad4b3@mail.gmail.com>
Date:	Fri, 14 Mar 2008 12:08:16 -0400
From:	Yan <rottled@...il.com>
To:	"Arjan van de Ven" <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: 2.6.22.6 - Discrepancy between running and on-disk kernels

Can you point me to where in the source that self-patching happens? I
tried grep'ing
through the source for anything relevant, and came out empty handed.

-yan

On Fri, Mar 14, 2008 at 12:00 PM, Arjan van de Ven <arjan@...radead.org> wrote:
> On Fri, 14 Mar 2008 10:51:43 -0400
>  Yan <rottled@...il.com> wrote:
>
>  > Hello,
>  >
>  > I have been trying to compare the code from the on-disk compressed
>  > kernel that was booted and the running kernel extracted
>  > from /dev/kmem. I extracted the kernel's code from the disk image by
>  > stripping the head.S and similar and gunzipping it, and extracted the
>  > kernel from /dev/kmem by reading data between _text and _etext symbol
>  > offsets.
>  >
>  > I then ran both through a disassembler and diff'ed the outputs.
>  > Predictably, the disassembly was similar, but not identical. Some
>  > instructions (e.g. bts) had a 'lock' prefix, where as others had a
>  > 'nop' in its place.
>  >
>  > There were other differences with some instructions like mfence.
>  > Everything else matched just fine, the differences were mostly in
>  > memory-referencing instructions.
>  >
>  > My question is, what can be changing the kernel between being on
>  > static storage and being loaded?
>
>  the kernel code is self-patching, it gets modified to match your system
>  during boot time. So you cannot assume that the kernel on disk and the kernel
>  in memory are identical.
>  (Same goes for modules)
>
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